A conventional turbo fan engine uses the core engine to drive a bypass fan mounted near the engine intake. Fan blades on the bypass fan drive a bypass flow around the core engine which combines downstream with the core exhaust flow to provide propulsive thrust.
A casing assembly extends around the outside of the fan to provide an outer wall of a flow annulus through the fan. The fan blades themselves are not normally provided with blade platforms, and so a number of separate circumferential wall inserts or “annulus fillers” are mounted on the outside of the fan rotor disc, in-between the fan blades, to form the inner wall of the flow annulus through the fan.
The annulus fillers are typically mounted on the fan rotor disc using a hook arrangement, such as the one described in International Application PCT/GB93/00372 (published as WO93/21425). Here, each annulus filler is provided with a pair of hooks, which extend radially inwardly from the filler to engage correspondingly shaped hooks provided on the outer face of the fan rotor disc. The hooks on the filler must be maintained in axial engagement with the hooks on the fan rotor disc, and one or more separate thrust rings is typically provided for this purpose.
A hook-type mounting arrangement such as the one described in International Application PCT/GB93/00372 has several disadvantages:
Firstly, the need to form dedicated, load-bearing attachment features such as hooks on the outside of a forged fan rotor disc adds to the cost and complexity of manufacturing the fan rotor disc.
In addition, safely engaging the hooks with one another is difficult and potentially time-consuming because, in practice, the hooks tend to be obscured from view by the adjacent blades and by the annulus filler itself during assembly. Failure safely to engage the hooks increases the risk of annulus filler detachment under a centrifugal load during rotation of the fan.
During a bird strike or fan blade off (FBO) event, a fan blade may be deflected and apply a circumferential load to an adjacent annulus filler. Tests have shown that annulus filler inserts secured using hook style arrangements such as the one described in International Application PCT/GB93/00372 are also vulnerable to detachment under these circumferential loads.